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June, 2024

The strength and ductility of steels are influenced by the characteristics of grains that make up its microstructure and their interaction with each other. In this study, we used 3D X-ray diffraction on I12, Diamond Light Source to track the behaviour of individual grains during straining of a low-carbon steel used in vehicle body shells. We discovered that for grains exhibiting a soft orientation (those that deform easily) can experience a wide range of stress levels, whilst hard orientations (those that cannot deform easily) can only possess a narrow stress range. The stress on each grain, which determines how easily it will plastically deform, is newly shown to be affected by the orientation of its neighbouring grains, especially at low levels of strain. As the steel deformed, grains rotated to reduce these interactions. This was an important discovery as this finding can be used to explain why these steels have such high ductility and hence formability during component manufacture.  

Figure caption: From a 0.3 x 0.3 x 0.3 mm3 region within a steel sample, individual grains were tracked when unloaded (left) and when a load is applied (right). Each point represents a single grain, where its coordinate indicates its position, its diameter is scaled by grain volume, and colour representing its stress in the direction of the tensile axis. We can use this information to understand why the stress states of individual grains develop, which can explain mechanical properties.

Ball, J.A.D., Kareer, A., Magdysyuk, O.V., Michalik, S., Connolley, T., Collins D.M., " Revealing per-grain and neighbourhood stress interactions of a deforming ferritic steel via three-dimensional X-ray diffraction", Communications Materials 5 (2024) 27

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